The invention relates to a method and device for monitoring direction indicator lights of a vehicle trailer (trailer indicator lights) and, more particularly, to a device which, upon the failure of a trailer indicator light, generates an imminent failure signal in the towing vehicle. The imminent failure signal is generated by a significant change in the pulse frequency generated from a flasher pulse generator upon the switching on of a direction signalling switch when the electrical load of the flasher pulse generator, connected through the direction signalling switch, deviates from its nominal load.
A generic device as shown in Prior Art FIG. 1 is known from use in vehicles, e.g., those manufactured by the assignee of the present invention, having trailer coupling and electrical trailer plug-in connections. The device includes a flasher pulse generator for generating fixed clock frequencies which can be changed over as a function of its electrical load. The electrical load can be connected via a direction signalling switch and is formed at least by the indicator lights of the towing vehicle. A detection device detects whether a trailer is electrically connected or not and generates a corresponding electrical detection signal. A switching device, which is controllable by the electrical detection signal, automatically controls the flasher pulse generator in order to change it over to another pulse frequency on the failure of one of the connected trailer indicator lights. This change-over generates an imminent failure signal.
In the case of a series installation of the known device, the nominal load of a flasher pulse generator can be changed over by a detection device to trailer operation. The pulse frequency of the flasher pulse generator can be changed as a function of the effective load. This means that there are additional indicator lights to control and to monitor. The detection device electrically detects whether a trailer is electrically connected or not. The trailer indicator lights then form a part of the effective electrical load of the flasher pulse generator so that even the failure of an indicator light in the trailer is imminently signalled by a significant frequency change of the pulse generator. As a result, an electrical switching device is formed for automatically switching over the frequency generated by the flasher pulse generator on the failure of one of the connected indicator lights of the trailer. The electrical switching device is electrically controllable by the detection device.
A necessary prerequisite for this operation, however, requires that at least one signalling line be laid between the detection device and the flasher pulse generator. Via the signalling line, an unambiguous detection signal for switching over the nominal load of the flasher pulse generator is conducted. The detection device is looped into the brake light current path to the trailer. The detection device detects, already on the electrical connection of the trailer provided with at least two brake lights to the towing vehicle, the different potential difference "potential-free" or "circuit connected via brake lights to ground." Detection devices of this kind are also known from DE 35 22 481 C1 and DE 30 38 464 C2.
Since automatic monitoring of the trailer indicator lights is legally prescribed, the above-mentioned signalling line must be laid between the then newly installed detection device and the existing flasher pulse generator, even when subsequently installing a trailer device. This was done so that the flasher pulse generator, as previously mentioned, can be changed over to trailer mode. However, this is time-consuming because the flasher pulse generator is located, for production reasons, in the front part of the vehicle while the detection device is installed in the rear part of the vehicle nearer to the mechanical trailer device.
Other types of monitoring circuits for the indicator system of a towing vehicle and trailer are also known from DE 27 53 528 A1 and DE 36 22 567 A1.
One known device indicates both the presence of an electrically connected trailer and the functioning of the indicator lights of this trailer by means of a control lamp which can be switched on in different operating modes. However, the frequency of the flasher pulse generator is not changed.
Another known device operates such that, on the failure of an indicator light of the towing vehicle or of the trailer, a control lamp which is otherwise switched on when the direction indicator signal is switched on, does not light up.
A third known test device described in DE 35 31 560 C2 for the indicator light signalling system of motor vehicles provides two control lamps. One control lamp indicates a failure of a towing vehicle indicator light and the other a failure of a trailer indicator light.
There is therefore needed a method for monitoring direction indicator lights of a vehicle trailer which dispenses with a nominal load change-over of the flasher pulse generator to trailer mode while maintaining the generation of an imminent failure signal by means of clock frequency change-over of the flasher pulse generator. Further, there is needed a device of a generic type, in particular for carrying out the above method in such a way that the subsequent installation of the electrical part of a trailer device in a towing vehicle can be simplified.
These needs are met by the present invention wherein operation of the trailer indicator lights occurs via an auxiliary switching device clocked with the frequency of the flasher pulse generator. The trailer indicator lights provide the load of a separate voltage supply. A fault switching signal is generated by a trailer indicator lights monitoring circuit not connected to the electrical load of the flasher pulse generator on the electrical detection of the failure of one of the trailer indicator lights and with the direction signalling switch switched on. Electrical isolation is achieved for at least one indicator light of the towing vehicle from the electrical load of the flasher pulse generator in the presence of the fault switching signal of the monitoring circuit for the purpose of simulating a deviation of the electrical load of the flasher pulse generator from the nominal load. Auxiliary operation of the isolated indicator light of the towing vehicle is achieved by means of the auxiliary switching device.
The method of the present invention simulates, upon the failure of one of the trailer indicator lights, a failure of a towing vehicle indicator light which, of course, is on the same side of the car-trailer combination as the failed trailer indicator light. Thus a load change occurs at the flasher pulse generator, due to which the latter then also generates a frequency which deviates significantly from the normal flasher frequency if the trailer indicator lights are not part of its electrical nominal load. A change-over of the flasher pulse generator to trailer mode with increased nominal load is thus made unnecessary. However, because the isolated towing vehicle indicator light is actually intact, it can continue to be operated via the same auxiliary switching device which pulses the trailer indicator lights. More specifically, it can be operated with the flasher pulse generator frequency provided on the failure of a trailer indicator light and switched on automatically. The vehicle user who searches for the source of the error can, therefore, clearly recognize with the direction signalling switch switched on the failure of the trailer indicator light.
The above-mentioned approval regulation is thus satisfied.
It should also be noted that the expression "separate" voltage supply was selected in order to make clear with a positive formulation that the trailer indicator lights are not a load of the flasher pulse generator. However, this only means that a separate positive terminal is provided for the trailer indicator lights and not that a voltage supply or source which is independent of the vehicle voltage source, e.g. accumulator or battery, has to be provided.
The present invention further provides a device including the connection of the trailer indicator lights to a separate voltage supply via a clocked auxiliary switching device, which can be switched on by means of the direction signalling switch in the presence of the detection signal. A monitoring circuit is provided for the trailer indicator lights for detecting a failure of one of these indicator lights in the presence of the detection signal and for generating a corresponding fault switching signal. Electrical isolation of at least one indicator light of the towing vehicle, switched on at the same time as the failed trailer indicator light, occurs from the electrical load of the flasher pulse generator upon the switching on of the direction signalling switch and in the presence of the fault switching signal of the corresponding monitoring circuit. As a result, the flasher pulse generator is controlled so as to change over the pulse frequency generated by it. Electrical connection of the isolated indicator light from the load of the flasher pulse generator to the separate voltage supply occurs via the auxiliary switching device.
In a very simple manner, the electrical isolation of the aforesaid towing vehicle indicator light from the electrical load of the flasher pulse generator can be connected by means of a break contact of a relay which is looped into the current path between the flasher pulse generator and this indicator light and which is controlled by the switching device.
Advantageously, this relay also has a make contact actuated at the same time as the break contact. The make contact connects the isolated indicator light to the clocked auxiliary device immediately after the isolation.
Of course, the relay can also merely have a simple three-way contact which alternately connects the downstream indicator light to the flasher pulse generator or to the auxiliary switching device.
The auxiliary switching device can contain, in an advantageous manner, a further relay which is clocked by the flasher pulse generator itself. Preventive measures will be taken to ensure that the electrical load of this relay does not bring about a frequency change-over. This further relay intermittently connects through its make contact the isolated indicator light to a separate voltage supply.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.